1. Field of the Invention
The present invention relates, in general, to linear positioning devices.
2. Description of the Relevant Art
Linear positioning devices have been employed to advance a work member an incremental amount upon each repeated energization of the positioning device. Numerous configurations for such linear positioning devices have been devised for different applications and some include a plurality of coils and movable cores for variable, discrete advance of a movable core depending upon the magnitude of the electrical current applied to the coils or the number of coils which are energized. Other incremental positioning devices simply operate in an on/off mode, that is, the movable member moves to one position when the coil is energized and then returns to the start or home position when current is removed from the electrical coil.
As disclosed in U.S. Pat. Nos. 4,835,425 and 5,055,725 issued to the Applicant, a typical linear positioning device utilizes an electrical coil and a magnetic core including at least one movable core member which is attracted to another portion of the core by magnetic flux induced in the core when electric current is applied to the coil. The core and coil extend along the total travel distance of a movable pole member and magnetic plunger which coaxially extend through the core and coil. Opposed faces of the movable pole member and the plunger are separated by an air gap. A biasing spring is connected between the plunger and the pole member to bias the pole member away from the plunger when electrical current is removed from the coil to re-establish the air gap between the pole member and the plunger for the next sequential energization of the coil. Latches act on the pole member and the plunger to provide unidirectional movement of the pole member and the plunger when a magnetic field is induced in the core which causes advance of the plunger into contact with the pole member.
However, it has been found that the linear motor disclosed in the aforementioned patent and pending application is effectively limited to small amounts of total advance of the pole member or output element attached thereto. This is due to the fact that the core and coil are constructed in such a length so as to extend along the total advance distance of the pole member or output element. For long distances, i.e., five to ten feet or more, this arrangement results in a linear motor having a high weight and cost due to the lengthy core and coil.
Further, while these linear motors are effective for small total advance distances and increments, the strength of the magnetic field in such linear motors is not constant over the entire length of the motor and falls off or decreases near the ends of travel due to the geometry of the field and the plunger. This introduces a variation into the strength or pull of the magnetic field over the overall travel distance of the linear motor.
Thus, it would be desirable to provide a linear motor for incrementally advancing a movable output member in a stepwise fashion upon repeated energization of the motor which overcomes the problems associated with previously devised linear motors. It would also be desirable to provide a linear motor which incrementally advances an output member over a long total distance of travel in successive incremental amounts. It would also be desirable to provide a linear motor which provides a constant incremental advance of an output member over the entire length of travel of the output member. Finally, it would be desirable to provide a linear motor which incrementally advances an output element or member and which has a reduced weight and manufacturing cost compared to previously devised linear motors.